1. Field of the Invention
The present invention relates to an electronic filing apparatus such as an apparatus for reproducing information from an optical disk on which information is recorded by mark length recording.
2. Description of the Related Art
In order to record information on an optical disk, a laser beam is focused into a fine light spot having a diameter of about 1 .mu.m and modulated in accordance with information to be recorded, thereby thermally changing the state of a recording film on the surface of the optical disk to perform recording. Reproduction of information is performed by optically reading changes in light reflected by recorded pits (state change portions).
In this case, an optical head is used to perform recording or reproduction of information with respect to an optical disk. This optical head is constituted by, e.g., a semiconductor laser oscillator for generating a laser beam, an objective lens for guiding a laser beam from the semiconductor laser oscillator to an optical disk, and a converter for converting changes in light guided by the objective lens and reflected by pits formed on the optical disk into electrical signals (analog signals).
Focusing or tracking control is performed by adjusting a position of the objective lens relative to the optical disk.
A reproduction circuit is constituted by an amplifying circuit for amplifying the analog signals from the converter of the optical head, and a binarizing circuit for binarizing the amplified analog signals from the amplifying circuit at a predetermined slice level.
Two types of recording methods, i.e., mark position recording and mark length recording are used in such an optical disk.
In the mark position recording, recording is performed by expressing recording information by a length between pits formed on an optical disk to perform recording. In this case, recorded information is reproduced by using a signal which is switched in accordance with a predetermined position of each pit. This predetermined position is a leading edge portion, a trailing edge portion, or a central position of a pit.
In the mark length recording, recording is performed by expressing recording information by a width (length) of a pit and a distance (space length) between pits. In this case, recorded information is reproduced by detecting the pit width and the distance between pits. More specifically, recorded information is reproduced by using a signal which is switched in accordance with both of leading and trailing edge portions of each pit.
In order to record a certain amount of recording information, a larger number of small-diameter pits must be formed in an optical disk using the mark position recording than in an optical disk using the mark length recording. Therefore, a laser optical system of an optical disk using the mark position recording is complicated. In addition, since a frequency band of analog signals is high in an optical disk using the mark position recording, its analog circuit such as an amplifying circuit is complicated.
Therefore, in order to perform high-density recording using an optical disk, the mark length recording is adopted.
when the mark length recording is used, a reflected light amount is changed (decreased) due to changes in focus offset, film sensitivity, and laser intensity. Therefore, a ratio of a pit width to a length between pits is largely shifted from a proper value. That is, the level of an analog signal in the binarizing circuit becomes lower than a proper level. Therefore, if binarization is performed at a predetermined threshold level, a signal indicating the leading and trailing edge portions of a pit is switched at a position shifted from an actual position.
As a result, since correct detection of a pit width (correct detection of both the leading and trailing edge portions of a pit) and correct detection of a length between pits become difficult to perform, erroneous reproduction is caused.